The major goal of this proposal is to determine how Arf, a potent tumor suppressor, plays an essential role during eye development to prevent a severe developmental eye disease. During mammalian eye development, the hyaloid vascular system (HVS) supplies nutrients to the developing eye and then completely involutes. Failed HVS regression is an integral component of a human eye disease known as persistent hyperplastic primary vitreous (PHPV), characterized by retrolental fibrovascular tissue in the vitreous, lens degeneration with cataract formation, and retinal detachment and dysplasia. Molecular mechanisms regulating HVS regression and the etiology of PHPV are largely unknown. However, occasional clinical reports of familial PHPV suggest that it may have a genetic basis. Recently, we have made the unexpected discovery that the Arf tumor suppressor gene is essential for the normal development of the mouse eye. Specifically, the HVS fails to involute and eye abnormalities that closely mimic PHPV develop in the first weeks of life in Arf-deficient mice. The pattern and timing of Arf expression during mouse eye development suggest that it may play a primary role in promoting HVS regression. The function of the Arf gene product during eye development appears to be independent of its known "down-stream" genetic target, p53. These findings provide the first evidence that Arf regulates eye development and that Arf may be an essential mediator of HVS regression to prevent a PHPV-like eye disease. We will couple in vivo studies of Arf-/- mice and new genetically-engineered mouse strains with in vitro studies to elucidate the mechanisms by which Arf promotes normal HVS regression in the developing eye. Specifically, our experiments will (1) identify and characterize the Arf-expressing cells in the embryonic and postnatal eye; (2) determine whether it acts in a cell-autonomous manner to promote normal eye development; and (3) elucidate the cellular and molecular effects of Arf that are required for vitreous maturation and HVS involution. The successful completion of our experiments will provide new insight into cellular and molecular mechanisms driving HVS regression in the eye. This should augment our knowledge of Arf function and begin to clarify the molecular pathogenesis for PHPV and potentially other vascular eye diseases.